This invention will be described in connection with embodiments especially adapted for use in exhaust lines from various types of engines, e.g., internal combustion engines of the spark ignited or compression ignited types, stationary or mobile, or gas turbines. It will be understood that the converters of the present invention may be used to effect various chemical reactions, particularly those occurring in fluid streams, especially gas streams, and which reactions are catalyzed or uncatalyzed. A particular reaction is oxidation of pollutant materials contained in exhaust streams from internal combustion engines.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated 8 Dec. 1987 to Cornelison et al which discloses a process for making precoated corrugated thin metal strips in a continuous manner, and accordion folding them into predetermined shapes; U.S. Pat. Nos. 4,152,302 dated 1 May 1979, 4,273,681 dated 16 Jun. 1981, 4,282,186 dated 4 Aug, 1981, 4,381,590 dated 3 May 1983, 4,400,860 dated 30 Aug. 1983, 4,159,120 dated 28 May 1985. 4,521,947 dated 11 Jun. 1985, 4,647,435 dated 3 Mar. 1987, 4,665,051 dated 12 May 1987 all to Nonnenmann alone or with another and which disclose multicellular honeycomb converters with corrugated and flat thin metal strips all brazed together; U.S. Pat. No. 5,070,694 dated 10 Dec. 1991 to Whittenberger which discloses spirally wound converters with corrugated strips and flat strips. International PCT Publication WO 90/12951 published 9 Apr. 1990 seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 to Kannainian et al. Reference may also be had to International PCT Publication No. 92/13626 filed 29 Jan. 1992. This application relates to a multicellular honeycomb converter body along an axis of which fluid can flow through a plurality of channels. The honeycomb has at least two discs in axially spaced relation to each other. According to the specification, there is at least one bar type support near the axis by which the discs are connected together and mutually supported. The invention is said to make possible design of the first disc for fast heating up through hot exhaust gas passing through, or applied electrical current. The honeycomb body serves as a support for catalyst in the exhaust system of an internal combustion engine. Another reference is German Patent Application 4,102,890 A1 filed 31 Jan. 1991 and published 6 Aug. 1992. This application discloses a spirally wound corrugated and flat strips combination wherein the flat strip contains slots and perforations and is electrically heatable. The flat strips include a bridge between leading and trailing portions. Groups of strips are separated by insulation means. Another reference is U.S. Pat. No. 5,102,743 dated 7 Apr. 1992. This patent discloses a honeycomb catalyst carrier body of round, oval, or elliptical cross-section including a jacket tube and a stack of at least partially structured sheet metal layers intertwined in different directions in the jacket tube. The stack has a given length and a given width. At least one of the sheet metal layers has a greater thickness over at least a part of one of the dimensions than others of the layers. Such at least one layer is formed of thicker metal or of a plurality of identically structured metal sheets in contiguous relation.
A common problem with many of the prior devices has been their inability to survive severe automotive industry tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at a high temperature (between 800 and 950 degrees C.; 1472 to 1742 degrees F., respectively) with exhaust gas from a gas burner or a running internal combustion engine simultaneously passing through the device. If the device telescopes, or displays separation or folding over of the leading or upstream edges the foil leaves, or shows other mechanical deformation or breakage up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test.
The Hot Cycling Test is run with exhaust flowing at 800 to 950 degrees C.; (1472 to 1742 degrees F. ) and cycled to 120 to 200 degrees C. once every 10 to 20 minutes for 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips, or mechanical deformation or breakage is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests" and have proved very difficult to survive. The structures of the present invention will survive these Hot Tests.
In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV."
Another stainless steel metal alloy especially useful herein is identified as Haynes 214 alloy which is conunercially available. This alloy and other useful nickeliferous alloys are described in U.S. Pat. No. 4,671,931 dated 9 Jun. 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation and high temperatures. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon, and steel making impurities. Haynes 230 alloy, also useful herein has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, a trace amount of lanthanum, balance nickel.
The ferritic stainless steels, and the Haynes alloys 214 and 230, all of which are considered to be stainless steel, are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metal alloys that are useful for use in making thin metal strips for the multicellular honeycomb converter bodies, or monoliths, hereof. Suitable metal alloys must be able to withstand "high" temperatures, e.g., from 900 degrees C. to 1200 degrees C. (1652 degrees F. to 2012 degrees F. ) over prolonged periods.
Other high temperature resistive, oxidation resistant metal alloys are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used as "thin" metal strips or layers, that is, having a thickness of from about 0.0005" to about 0.005", and preferably from 0.0015" to about 0.0037".
It has now been found that a preferred mode of forming a multicellular honeycomb converter body of a plurality of thin metal strips is to secure those strips at the central portions thereof and then to wind about a welded rigid center to form the honeycomb body. Reference may be had to the commonly owned application of David Thomas Sheller, Ser. No. 08/370,643 filed 10 Jan. 1995 and entitled "Reinforcing Web for a Multicellular Converter" which details the construction of such a multicellular converter. The process of the present invention provides a novel and effective way to secure the thin metal strips forming the honeycomb body into a rigid central post to facilitate winding about the welded or brazed center and to prevent axial shift of the thin metal leaves or strips of the core in response to the rigors of the Hot Tests described above.